The code is attempting to call an insert function that does not exist in MyBBEditor, instead the function is named insertText. Renaming the two occurences of insert in the js files will fix the problem. I assume this bug is not too much of an issue on mainline MyBB forums, but it certainly is one on a frankenstein MyBB like Disco's.

It is most likely we are one of the rare forums encountering this as we are using YuiEditor instead of the mainline ScEditor, as insert apparently still exist in ScEditor.

Thanks for the report. It should work better now.
I didn't go for that exact solution, but it certainly seems to be the source of the problem.

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Please only PM me through the forums for things that need to be private like anticheat etc.
If I am in a Discord channel, chances are I won't read anything unless @'d individually (most group pings don't seem relevant to me).
FLServer/FLHook works in mysterious ways - or doesn't work, as the case may be.